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<h1>Data Runs</h1>

<p>Bodies of non-resident attributes are stored on disk in one or several cluster chains referred to as runs. A run is a sequence of adjacent sectors characterized by the number of its initial cluster and length. A set of runs is referred to as run-list.</p>

<p>For space saving purposes, the run length and the initial cluster number are stored in fields of variable size. Thus, if run size fits into a byte (i.e. its value does not exceed 255), then it is stored in a byte. Consequently, if a run size requires a long word for its presentation, then it is stored in a long word.</p>

<p>The size fields are stored in 32-bit fields referred to as nibbles. Hexadecimal system allows easy conversion of bytes into nibbles and vice versa. Lowest nibble corresponds to the lowest hexadecimal bit in a byte, the highest nibble corresponds to the highest one. E.g., 69h consists of two nibbles – the lowest one is 9h, the highest – 6h.</p>

<p>A list of data runs is an array of structures; each of them describes the characteristics of its own run, and in the list end there is a 16-bit end marker equal to 00h. The first byte of the structure consists of two nibbles: the lowest one defines the field length for the initial run cluster (conventionally designated as F), the highest nibble determines the number of clusters in that run (L). Run length field follows next. Depending upon the L value, it can take from 1 to 8 bytes (longer fields are not allowed). The first byte of the initial file cluster field is located at offset 1&nbsp;+&nbsp;L bytes from the structure beginning (which corresponds to 2+2*L nibbles). </p>

<p><strong>Table </strong> Data run structure</p>

<p><img src="Data%20Runs_files/data_run.gif"></p>

<h6><img src="Data%20Runs_files/data_ruo.gif"></h6>

<h6>Example of a $DATA attribute </h6>

<p>Let us examine a run-list corresponding to a normal unfragmented file shown in the figure above: <img src="Data%20Runs_files/data_rup.gif">. </p>

<p>The first byte is&nbsp;22h. Lowest nibble (x2h) describes the size of a run length field, the highest one (2xh)&nbsp;– the initial cluster field size. The next few bytes represent the run length field, which in this case takes two bytes – 79h 21h. Two other bytes (26h&nbsp;0?h) define the number of the initial run cluster. The zero byte at the end (00h) indicates that its is the last run in the file. As a result, our file consists of a single run, which starts with cluster 0?26h (reverse byte order) and ends with cluster 0?26h +&nbsp;2179h&nbsp;=&nbsp;2D9Fh.</p>

<p>If the file were fragmented, the run-list could look, for example, as follows: <img src="Data%20Runs_files/data_ruq.gif">. The first run (run&nbsp;1) starts with cluster 342573h and continues until cluster 342573h&nbsp;+&nbsp;38h&nbsp;=&nbsp;3425ABh. The second run (run&nbsp;2) starts with cluster 0211E5h and continues until cluster 0211E5h&nbsp;+&nbsp;114h&nbsp;=&nbsp;212F9h. The third run (run&nbsp;3) starts with cluster 0300AAh and continues to cluster 0300AAh&nbsp;+&nbsp;42h&nbsp;=&nbsp;300ECh. The zero byte at the end (00h) indicates that it is the last run in the file. Thus, the file consists of three data runs placed on disk as follows: 342573h – 3425ABh; 0211E5h – 212F9h; 0300AAh – 300ECh. </p>

<p>Beginning with version 3.0, NTFS supports sparse attributes, i.e. those which do not write to disk clusters containing zeroes only (if an attribute is sparse, then its flag will be 8000h). The field of the initial cluster number of such run may be equal to zero meaning that no cluster has been assigned to that run. The length field contains the number of clusters filled with zeroes. They should not be read from disk. You should create them manually in memory. <br>
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